1. Technical Field
This invention relates generally to methods for photochemical reactor characterization. More particularly, this application is related to a method of using dyed microspheres exposed to UV radiation to measure the dose distribution delivered by the reactor for any given set of operating conditions.
2. Background Information
Ultraviolet (UV) systems have been used for the treatment of aqueous liquids, particularly in disinfection applications. In the past, several thousand UV based disinfection systems have been installed and placed into operation throughout the United States and Europe. These systems are an alternative to conventional chlorine-based disinfection and their emergence may be attributed to their comparative small size, low cost, simplicity, antimicrobial efficacy, and “environmental friendliness” relative to existing chlorine-based systems.
Traditionally, the application of UV irradiation for disinfection has been directed to conventional wastewater disinfection. More recently, this application has expanded to include wastewater reuse and potable water supplies. Discoveries of the effectiveness of UV irradiation for inactivation of protozoan (oo)cysts suggest that UV irradiation could be used with increasing regularity as a disinfection alternative.
UV irradiation has also been shown to be effective for the inactivation of the spores of Bacillus anthracis, thereby suggesting that UV irradiation could also be used for maintaining the security of public water supplies against intentional or unintentional contamination by biological warfare agents.
UV reactors used in treatment operations deliver a distribution of UV doses to particles that pass through them, where the UV dose is defined as the time-integral of UV intensity history over the period of exposure. The variation in the dose delivery within such a system, even when operated under steady state conditions, is attributable to the heterogeneity in the radiation intensity field and turbulence within the flow field. The efficiency of photochemical reactors, including those used for disinfection of water, has been shown to be governed by the distribution of UV doses delivered to particles that traverse the irradiated zone of these systems.
Efforts to characterize the performance of UV systems have involved using estimates of the average UV dose delivered by the reactor. Although several methods exist for estimation of “average UV dose”, all suffer from the fact that they represent gross oversimplifications of reactor performance.
One of the problems with using an “average UV dose” is that any reactor modifications relating to improvements in the performance of the process or the reactor efficiency are minimized or in sometimes not included in the analysis of the performance of the UV system. For example, one approach has demonstrated that the inclusion of internal mixing components, in otherwise conventional UV system configurations, can yield increases in antimicrobial efficacy of 100% or more. These modifications, however, result in minimal, if any, change in the “average UV dose” delivered by the UV system.
Currently, the only methods available to estimate the UV dose distribution involve using numerical simulations of process behavior, usually involving a component approach. The intensity field may be simulated using one of several available models. Computational fluid dynamics (CFD) are commonly used to represent the flow field, including its turbulence characteristics. The results of these simulations are integrated to provide a representation of the dose distribution. Most such simulations employ a Lagrangian (particle centered) approach, in which a particle-tracking algorithm is used to interrogate the simulated flow field and simulate individual particle trajectories. In a Lagrangian modeling approach, dose increments are assigned to each particle step within its simulated trajectory, such that an estimate of particle-specific dose may be assigned to each simulated particle trajectory. By repeating this process for a large population of particles, it is possible to close on a stable dose distribution estimate.
Among the methods used to estimate average UV dose, the two most common involve a simple numerical simulation and a set of experimental measurements, respectively. The first method involves numerical approximation of the product of the spatial average of radiation intensity within the reactor (Iavg) and the mean hydraulic detention time (θ). This method of the reactor characterization is simple to complete but yields unreliable, often misleading predictions of reactor performance.
The second method of reactor characterization is known as biodosimetry. In biodosimetry, a challenge organism is imposed on an actual continuous flow reactor. Reactor performance is quantified by measuring the concentration of the viable challenge organisms in the influent and effluent streams. The UV dose-response behavior of the challenge organism is measured over a range of UV doses using a collimated beam with a shallow, well-mixed batch reactor to accomplish irradiation. The effective dose, which is sometimes referred to as the “reduction equivalent dose,” is defined as the dose delivered by the collimated beam that accomplishes the same extent of inactivation as the follow through reactor.
Biodosimetry is the most commonly applied method for reactor characterization and has been defined as an acceptable method of reactor validation for potable water UV disinfection systems in the United States and much of Western Europe. It is also well known, however, that the results of biodosimetry are not useful for quantitative prediction of the inactivation of waterborne microorganisms, other than those used as challenge organisms. Therefore, it is difficult to obtain an accurate, quantitative prediction of microbial inactivation from biodosimetry results for any organism that has UV dose-response behavior different from the challenge microorganism(s).
Furthermore, biodosimetry provides no information about the dose distribution. Without this information, it is not possible to develop an accurate prediction of microbial inactivation for the wide spectrum of waterborne microorganisms that could be imposed on a UV system. In spite of this shortcoming, biodosimetry exists as the standard method for characterization of UV reactors. Biodosimetry has been the most widely used tool for UV system characterization, particularly given the skepticism that accompanies reactor characterization by purely numerical methods, such as the Lagrangian model discussed above. Limitations of biodosimetry have been recognized for many years. The development of standardized techniques for dose characterization has been identified as a top research priority among researchers involved in UV disinfection.
Another important shortcoming of conventional photochemical reactor design and analysis has been the lack of a mechanism or protocol for estimating the distribution of UV doses delivered by the system. In reality, the three-dimensional nature of the velocity and intensity fields of these systems dictates that all continuous-flow UV systems will deliver a broad distribution of doses.
With the recent discovery of the effectiveness of UV irradiation for disinfection of drinking water, there is need for the development of methods of reactor validation and testing. Currently, there is tremendous interest in the application of UV irradiation as a disinfection process for the production of drinking water, although relatively few utilities currently employ UV-based processes. Reactor validation and testing are accomplished using relatively crude, empirical methods, such as biodosimetry. These methods provide index measurements of reactor behavior, but cannot be used to make quantitative predictions of reactor performance or efficiency.
There is also need for practical tools to assess the efficiency and reliability of a UV system available to UV system designers, regulators and treatment facility operators. These tools should be universal and enable comparisons of different UV systems as well provide for accurate and reliable predictions of inactivation of all waterborne microorganisms.
Although, several experiment-based methods are available for characterization of UV reactors, as discussed in the preceeding paragraphs, these methods do not yield an estimate of the dose distribution delivered by the system. In most instances, the reactor performance is characterized using a grossly over simplified representation of reactor behavior. Consequently, existing experimental methods do not yield information that may be used for quantitative assessment of photochemical reactor performance. Thus, there is a need for a method to measure the UV dose distribution delivery in photochemical reactors that can provide accurate assessment of photochemical reactor performance.